The present invention relates to a mass spectrometer and, more particularly, to an ion source and a mass spectrometer having a beam extractor electrode system which is suitable for extracting a beam having a small angular dispersion and a high transmission characteristic from an ion source.
FIG. 7 is a cross-sectional view showing an ion source and a beam extractor electrode system in a conventional mass spectrometer. In a case of an ion source of electron bombardment ionization type, the ions generated by irradiation of electrons in an ionization chamber 1a are repelled with a repeller electrode if to which a positive voltage several volts higher than the voltage of the ionization chamber 1a. The ions are applied and pushed out through an extractor slit 15 of an acceleration electrode 1b. Since the ion beam 2 pushed out is dispersed after crossing at a cross-over point 2c, the ion beam 2 is converged by providing a focussing electrode 1c and accelerated between the acceleration electrode 1b. and a ground electrode 1d up to approximately 6 kV. During that period, in order to effectively guide the beam to an analyzing part, that is, a sector magnetic field and a sector electric field, the width and the dispersion angle of the beam 2 are controlled by adjusting the voltage of the focussing electrode 1c. Practically, at starting of operation of a mass spectrometer, the amount of ion current passing through a slit 1e is measured by inserting an ion current monitor 8a utilizing a Faraday cup, a multiplier and a channel plate into a region of the ion beam passing through. The voltage 12f applied to the converging electrode 1c is adjusted so that the current signal 12a becomes maximum. The prior art is described in Japanese Patent Application Laid-Open No. 57-27553 (1982).
In the above conventional technology, generally, the gap between the acceleration electrode 1b and the focussing electrode 1c is approximately 10 times as large as the width of the slit 15, and change in dispersion angle of the beam 2, that is, the positional change of the cross-over point 2c, against change in the electrode voltage is extremely small. However, the optical system of a mass spectrometer is required to keep the width of beam within a certain value as well as the dispersion angle. Therefore, in the conventional construction of electrodes in which only one of electrode voltage 12f is adjustable, it is difficult to perform such an adjustment as to satisfy the limitations of, controlling both the angle of dispersion and the width of beam at the same a time.